DIDACTICAL STAND FOR THE STUDY OF THERMOELECTRIC GENERATORS

Transcription

1 DIDACTICAL STAND FOR THE STUDY OF THERMOELECTRIC GENERATORS DIDACTICAL STAND FOR THE STUDY OF THERMOELECTRIC GENERATORS Eng. Crina BOBEAN, Eng. Valentina PAVEL, Prof. Eng. Ioan VADAN PhD, Eng. Tudor ISOC, Eng. Codrin SOMESAN Technical University of Cluj Napoca, Romania, Electrical Engineering Faculty. REZUMAT. Această lucrare prezintă un stand didactic pentru studiul generatoarelor termoelectrice, dispozitive pentru conversia directă a căldurii în energie electrică. Deşi au eficienţă scăzută, sub 0%, utilizarea lor în generarea de energie este utilă datorită altor avantaje: complexitate redusă, lipsa părţilor în mişcare şi a întreţinerii. Lucrarea prezintă metodologia pentru studiul,, testarea şi identificarea caracteristicilor energetice ale generatoarelor termoelectrice,, precum şi proiectarea unui stand experimental ental pentru efectuarea acestor teste. Cuvinte cheie: generatoare termoelectrice, testare. ABSTRACT. This paper presents a stand for didactical study of the thermoelectric generators. Thermoelectric generators are devices that directly convert heat into o electricity. Although their efficiency is low, below 0%, their use in power generation is beneficial due to other characteristics: reduced complexity, lack of moving parts and therefore the maintenance.the paper presents the methodology for studying,, testing and power characteristics identification of the thermoelectric generators and the design of an experimental stand for performing the tests. Keywords: thermoelectric generators, testing.. INTRODUCTION Thermoelectric conversion is a technology that allows direct conversion of heat into electricity. Since a thermoelectric generator operates between two heat sources: a warm temperature T and a cold temperature T, their effectiveness is limited by the second principle of thermodynamics (Carnot efficiency): T η =. () T t However, thermoelectric generators operate at small temperature differences and have no moving parts and maintenance, that makes them very attractive for the production of electricity from heat sources with low thermal parameters as: recovery of waste heat from industrial processes (combustion gases from furnaces and boilers, water cooling of the compressors and electric furnaces etc.). heat recovery from exhaust pipes of vehicles. Thermoelectric generators can be used to produce electricity by direct conversion of renewable energy as: solar energy; geothermal energy; cogeneration plants based on wood. Also, due to their robustness, thermoelectric generators are suitable for special applications: airspace, military and telecommunications, cathodic protection of gas pipes, etc.. Manufacturing technology of thermoelectric generators has progressed greatly in recent years due to the high demand for thermoelectric refrigerators, knowing that powered with dc voltage, thermoelectric generators are reversible and can operate as heat pumps (refrigerator or air conditioning devices). The paper presents the operating principle of thermoelectric generators and a test methodology to identify their main characteristic parameters. For this purpose, a stand for the study of thermoelectric generator characteristics is designed and built. This stand can be used for educational purposes, to train students in energy conversion and renewable energy and for research purposes, in the case of doctoral students and researchers from industry. It is known that a high power thermoelectric generator will contain hundreds or thousands of lowpower thermoelectric modules (hundreds of watts). For design optimization it is recommended the experimental study of the behavior of a single module, or a small number of modules under thermal and electrical data, experimental results being extrapolated to high power thermoelectric generator design. Buletinul AGIR nr. /0 iunie-august 4

2 WORLD WORLD ENERGY ENERGY SYSTEM SYSTEM CONFERENCE CONFERENCE WESC - WESC 0 The technical characteristics of the thermoelectric module from this stand, initially were unknown. On the stand were performed experimental determinations, whose analysis allowed the identification of main thermal and electrical characteristics of the thermoelectric generator to be studied.. THERMOELECTRIC GENERATORS WORKING PRINCIPLE Thermoelectric generators (TEG) are compact devices that convert heat power directly into electricity due to the Thomson, Peltier and Seebeck effects. Seebeck effect (Figure ) was discovered in 8 by Thomas Seebeck. Fig.. The Seebeck effect principle []. Heating one junction of two wires made of different metals, one noted that a voltage is obtained in the circuit: ( ) T VAB = α A α B, () where α A and α B are the two metals Seebeck coefficients and T is the temperature difference between the two junctions []. The materials were classified as a generated voltage in relation to an ideal material, superconductor. In practice, lead is used as a reference scale. The Peltier effect is discovered in 84 by Jean Peltier and is actually a consequence of the Seebeck effect (Figure ). ( Π Π ) I QAB = A B, () where Π A and Π B are Peltier coefficients of the two metals; Q AB is the pump power and I is the current flowing through the circuit []. The Thomson effect was discovered by William Thomson in 854. It was first formulated the following relationship: dt q A = τ A I, (4) dx where q A is the heat absorption rate per conductor unit length; τ A is the Thomson coefficient for the conductor A and dt/dx is the temperature gradient per unit length of the conductor. The relationship is thermodynamically reversible and describes the behavior of a material []. Thomson has unified the three coefficients by the relation: dα τ = T, (5) dt Π = α T, (6) where τ AB is the Thomson coefficient; T is the absolute temperature (K) and dα/dt is the change of Seebeck coefficient with the temperature []. A typical thermoelectric generator (Figure ) consists of two dielectric parts (usually ceramic) that serve as support for N and P type small semiconductors, electrically connected in series and thermally in parallel. Thus the thermoelement requires a source of high temperature for the hot side and cooling for the cold side (environment, ventilation, cold water etc.). Fig.. The Peltier effect principle []. If a current source is introduced in the Seebeck thermoelement circuit, in the two junctions a power is produced and consumed, so we have a heat transport from hot source to cold source, a heat pump is made without motion parts: Fig.. The construction of a thermoelectric generator []. Components of N type and P type are made of different semiconductor materials (optimized for Seebeck effect), which have a different density of free electrons at the same temperature. As semiconductor materials are used: Bi Te, Sb Te and Bi Se. As the heat moves from the hot side to the cold side, charge carriers (electrons and holes) are carried with the heat. In this way a significant potential difference is generated (Seebeck voltage): U = α T h T ), (7) ( c 44

3 DIDACTICAL STAND FOR THE STUDY OF THERMOELECTRIC GENERATORS where α is the Seebeck coefficient; Th is the temperature of the hot source and Tc is the temperature of the cold source. Materials used for the thermoelements must meet the following requirements: a high Seebeck coefficient α to produce a high voltage; thermal conductivity λ as low as possible, to reduce the heat flow that bypasses the thermoelement; electrical conductivity σ as high as possible to reduce the ohmic resistance and the Joule loss in the thermoelement. These are contradictory requirements; metals have good electrical conductivity, but also good thermal conductivity. To assess the quality of a thermoelectric generator is used the so-called figures of merit Z or ZT: Fig. 4. Thermoelectric module. α σ Z=, λ ZT = (8) α σ T. λ η= Table Seebeck coefficients and merit figures for materials thermoelement selection at room temperature []. (Bi-xSbx)Te p-bite p-si Sb Ni-Cr (80/0) Cr Ni Cu-Ni Bi n-bite Bi(SexTe-x) -6 α [0 V/K] la Thermoelectric efficiency of such a generator can be expressed as follows: (9) Table shows the Seebeck coefficients and figures of merit for the main materials used in thermoelements construction, at room temperature. Element/compound To increase the power unit, several thermoelements are coupled in an ensemble by connecting them thermaly in parallel and electric in series (Figure 4). Thermoelectric generators are widely used in the following fields: military, medical, industrial, automotive, scientific laboratories, the opto-electric, telecommunications, microelectronics. They are also used to supply cathodic protection of gas pipes and spacecraft instruments. ασ/λ [0-K-] - (0). THERMOELECTRIC GENERATOR STAND STRUCTURE Figure 5 presents the thermoelectric generator to be studied It can be seen that the p-type silicon has the highest Seebeck coefficient, but the smallest figure of merit because it has low electrical conductivity and high thermal conductivity, therefore is used rarely, only at high temperatures. Because semiconductors are not working at very high temperatures, are obtained about 5% efficiency. Currently, there are started researches for the use of other materials that will improve the performance of thermoelectric generators. Generated Power. Transfered Heat Fig. 5. Thermo-electric generator TOG 7. Figure 6 presents the thermoelectric module designed and built around this thermoelectric generator. Fig. 6. The thermoelectric module: heat sink (radiator); thermoelectric generator; heat source ( resistors). Buletinul AGIR nr. /0 iunie-august 45

4 WORLD ENERGY SYSTEM CONFERENCE WESC 0 WORLD ENERGY SYSTEM CONFERENCE - WESC Figure 7 presents the stand for studying the thermoelectric generators. The thermoelectric module is heated with 4 ceramic resistors of 0.56Ω, 0W, connected in series to a 5V power source 4. At the output of thermoelectric assembly is used a load resistance. Measurement devices are used too, for voltage and current at the output of the generator. It is also used a thermal imager of type FLUKE Ti0 [4] for the measuring of the temperatures of the hot and cold side of the thermoelectric module. 4 After analyzing the measurements presented in Table it can be seen that the maximum power produced by the generator is 0. mw and it is obtained for a load resistance of 5 Ω, cold side temperature in this case is T=6 C and the warm side temperature is T= C, temperature difference is 70 C. In this Table it can be seen two series of measurements performed every two minutes. The hot side temperatures are increasing in the second series of measurements, but the increase of generated power is insignificant because the temperature diferences between hot side and cold side of thermoelectric generator have remained almost the same. In figure 8 is presented the image, taken by thermovision camera FLUKE Ti0, for the thermoelectric assembly at the beginning of the experiment (the measurement Nr. ), when the temperatures are not to high and the power produced is low. The generated power is only 4.5 mw, especially because of the discrepancy between the load resistance (0Ω) and the inner resistance of the thermoelectric generator (.9Ω). Fig. 7. Thermoelectric generator stand: Load (a calibrated resistor); Thermoelectric module; The thermal imager FLUKE Ti0; 4 - The power source. In Table are presented the measured parameters of the thermoelectric module: the output voltage and output current for the module, the hot side temperature T and cold side temperature T and also different values considered for the load resistance. Were also calculated the power produced by the generator and the temperature difference between the two parts of it. Table Mesured and calculated parameters for the thermoelectric generator Crt. no RS [Ω Ω] U [V] I [ma] P [mw] T [ C] T [ C] T [ C] Fig. 8. Infrared images and temperature profile for the thermoelectric module assembly at the beginning of the experiment: RS=0Ω, T=4 C, T=05 C, P=4.5mW. In figure 9 is presented the image at the end of experiment (the measurement Nr. ), when the temperatures are the highest from the cycle. 4 46

5 DIDACTICAL STAND FOR THE STUDY OF THERMOELECTRIC GENERATORS Fig. 9. Infrared images and temperature profile for the thermoelectric module assembly at the end of the experiment: R S =Ω, T =64 C, T =5 C, P=.87mW. In this case, the generated power is very low (.87mW), also because of the discrepancy between the load resistance (Ω) and the inner resistance of the thermoelectric generator (.9Ω). In this picture the temperature of radiator is not correct because the profile line passes between two fins. From the Table can be seen that the maximum generated power of P=0.8mW corresponds to the measurement Nr. 9, to a voltage of 0.6V and to the load resistance of 5Ω. These values can be considerred as rated power, rated voltage and rated load resitance. This rated load resistance is very closed to inner resistance of thermoelectric generator, as is stated in the electric circuit theory. This measurement, made in rated conditions, can be used for calculation of the Seebeck coefficient: U 0.8V α = = = V C. () (T T ) 70 C The thermoelectric module image and the temperature profile, coresponding to this optimal point of operation is presented in figure 0. Fig. 0. Infrared images and temperature profile for the thermoelectric module assembly at the maximum power generated (the rated power): R S =5Ω, T =6 C, T = C, P=0.8mW. From these measurements is difficult to calculate the efficiency of this thermoelectric generator because of the difficulties in evaluation of the heat transferred between the hot side and cold side and the total heat losses by convection and radiation. In order to estimate the energy efficiency of a thermoelectric generator, the measurementss must be accompanied by a Finite Element Method thermal analysis [5,6]. 4. CONCLUSIONS This paper presents a testing methodology for the identification of the main electrical and thermal parameters for an thermoelectric generator. It is also presented a designed thermoelectric module and a stand for performing the tests. For a small thermoelectric generator, the proposed tests are performed and analysed. Buletinul AGIR nr. /0 iunie-august 5 47

6 WORLD WORLD ENERGY ENERGY SYSTEM SYSTEM CONFERENCE CONFERENCE WESC - WESC 0 BIBLIOGRAPHY [] Engelke, K. W., Novel Thermoelectric Generator for Stationary Power Waste Heat Recovery, Master Thesis, MONTANA STATE UNIVERSITY, January 00, USA. [] *** Laird Technologies, Thermoelectric Handbook, Product Information, Assembly Information, Performance and Properties, 0. [] Chepko, C., D., Heat Energy Conversion, heatenergy.html. [4] *** Ti0 Thermal Imager Users Manual, umeng0000.pdf [5] Bitschi A., Modeling of thermoelectric devices for electric power generation, Doctoral thesis, Swiss federal Institute of Technology (ETH), ZURICH 009. [6] Fairbanks J., Thermoelectric developments for vehicular applications. Proccedings Of Diesel Engine Efficiency and Emission Research (DEER) Conference, DETROIT, MI, August 4, 006. About the authors Eng. Crina BOBEAN, PhD Student, Technical University of Cluj Napoca, Department of Power Engineering and Management. bobeanc@mail.utcluj.ro. She Graduated from Electrical Engineering Faculty of Technical University of Cluj-Napoca. After finishing University she worked as a design engineer for electrical networks at SC Power Design SRL. The PhD research topic is solar energy conversion in electricity using thermoelectric converters. Eng. Valentina PAVEL, PhD Student, Technical University of Cluj-Napoca. Department of Power Engineering and Management. tina_pavel@yahoo.com. She graduated the Technical University of Cluj-Napoca, Electrical Engineering Faculty. After graduation she started PhD studies, at the same University, with the topic: contributions to the development of the electrical energy storage systems with compressed air. Prof. Eng. Ioan VADAN, PhD, Technical University of Cluj-Napoca, Department of Power Engineering and Management. Ioan.Vadan@eps.utcluj.ro. Graduated at the Polytechnical University of Timisoara. After finishing of the university he worked 7 years at Mining Company Cluj, 0 years as research engineer at Technical University of Cluj Napoca. Now is professor at the same university, Department of Power Engineering and Management. Research topics: Power generation, Energy conversion, Electric heating. Eng. Tudor ISOC, PhD Student, Technical University of Cluj-Napoca, Department of Power Engineering and Management. tudor_isoc@yahoo.com. He graduated Technical University of Cluj-Napoca, Electrical Engineering Faculty. After graduation he started working for the Phd thesis, studying Oscillating Wind Generator. Eng. Codrin SOMESAN, PhD Student, codrin.somesan@somesan.ro. He graduated Technical University of Cluj-Napoca, Electrical Engineering Faculty. After graduation he started working for the Phd thesis, studying Solar photovoltaic parks. 6 48